This invention relates generally to the rendering of latent electrostatic images visible using multiple colors of dry toner of developer and, more particularly, to highlight color imaging and an improved cleaning system for removing residual toner from a charge retentive surface.
The invention can be utilized in the art of xerography or in the printing arts. In the practice of conventional xerography, it is the general procedure to form electrostatic latent images on a xerographic surface by first uniformly charging a photoconductive insulating surface or photoreceptor. The charge is selectively dissipated in accordance with a pattern of activating radiation corresponding to original images. The selective dissipation of the charge leaves a latent charge pattern on the imaging surface corresponding to the areas not struck by radiation.
This charge pattern is made visible by developing it with toner. The toner is generally a colored powder which adheres to the charge pattern by electrostatic attraction.
The developed image is then fixed to the imaging surface of is transferred to a receiving substrate such as plain paper to which it is fixed by suitable fusing techniques.
The concept of tri-level xerography is described in U.S. Pat. No. 4,078,929 issued in the name of Gundlach. The patent to Gundlach teaches the use of tri-level xerography as a means to achieve single-pass highlight color imaging. As disclosed therein, the charge pattern is developed with toner particles of first and second colors. The toner particles of one of the colors are positively charged and the toner particles of the other color are negatively charged. In one embodiment, the toner particles are supplied by a developer which comprises a mixture of triboelectrically relatively positive and relatively negative carrier beads. The carrier beads support, respectively, the relatively negative and relatively positive toner particles. Such a developer is generally supplied to the charge pattern by cascading it across the imaging surface supporting the charge pattern. In another embodiment, the toner particles are presented to the charge pattern by a pair of magnetic brushes. Each brush supplies a toner of one color and one charge. In yet another embodiment, the development system is biased to about the background voltage. Such biasing results in a developed image of improved color sharpness.
In tri-level xerography, the xerographic contrast on the charge retentive surface of photoreceptor is divided three, rather than two, ways as is the case in conventional xerography. The photoreceptor is charged, typically to 900 v. It is exposed imagewise, such that one image corresponding to charged image areas (which are subsequently developed by charged area development, i.e. CAD) stay at the full photoreceptor potential (V.sub.ddp or V.sub.cad, see FIGS. 1a and 1b). The other image is exposed to discharge the photoreceptor to its residual potential, i.e. V.sub.c or V.sub.dad (typically 100 v) which corresponds to discharge area images that are subsequently developed by discharged-area development (DAD). The background areas exposed such as to reduce the photoreceptor potential to halfway between the V.sub.cad and V.sub.dad potentials, (typically 500 v) and is referred to as V.sub.w or V.sub.white. The CAD developer is typically biased about 100 v closer to V.sub.cad than V.sub.white (about 600 v), and the DAD developer system is biased about 100 v closer to V.sub.dad than V.sub.white (about 400 v).
Various techniques have heretofore been employed to develop electrostatic images as illustrated by the following disclosures which may be relevant to certain aspects of the present invention.
As disclosed in U.S. Pat. No. 3,457,900, magnetic brushes have been designed to give fringe field or solid area development by adjusting the conductivity of the carrier. It is also stated therein that they can also be made to tone areas of less charge and clean areas of greater charge giving what is known in the art as a reverse development.
As discussed in U.S. Pat. No. 4,397,264 which relates to a conventional xerographic image development system, conductive magnetic brush (CMB) development and insulating magnetic brush (IMB) development systems suffer from limitations in their abilities to meet the full range of copy quality requirements. Specifically, insulating magnetic brush development systems have difficulty in using one developer roller to develop both fine lines and solid areas. In order to optimize solid area development with an insulating developer material, the spacing between the developer roller and photoconductive surface must be made quite small. However, low density fine line development occurs at a larger spacing to take advantage of the accuracy of fringe field development with insulating materials. This permits development with high cleaning fields so as to minimize background development.
As further discussed in the '264 patent, conductive magnetic brush development systems inherently fail to faithfully reproduce low density lines. Conductive developer materials are not sensitive to fringe fields. In order to achieve low density fine line development with conductive developer materials, the cleaning field must be relatively low. This produces relatively high background.
U.S. patent application Ser. No. 913,181 now U.S. Pat. No. 4761668 filed in the name of Parker et al and assigned to the same assignee as the instant application which relates to tri-level printing discloses apparatus for minimizing the contamination of one dry toner or developer by another dry toner or developer used for rendering visible latent electrostatic images formed on a charge retentive surface such as a photoconductive imaging member. The apparatus causes the otherwise contaminating dry toner or developer to be attracted to the charge retentive surface in its inter-document and outboard areas. The dry toner or developer so attracted is subsequently removed from the imaging member at the cleaning station.
U.S. patent application Ser. No. 78,750 now U.S. Pat. No. 4761672 filed in the name of Parker et al and assigned to the same assignee as the instant application which relates to tri-level printing discloses apparatus wherein undesirable transient development conditions that occur during start-up and shut-down in a tri-level xerographic system when the developer biases are either actuated or de-actuated are obviated by using a control strategy that relies on the exposure system to generate a spatial voltage ramp on the photoreceptor during machine start-up and shut-down. Furthermore, the development systems' bias supplies are programmed so that their bias voltages follow the photoreceptor voltage ramp at some predetermined offset voltage. This offset is chosen so that the cleaning field between any development roll and the photoreceptor is always within reasonable limits. As an alternative to synchronizing the exposure and developing characteristics, the charging of the photoreceptor can be varied in accordance with the change of developer bias voltage.
U.S. patent application Ser. No. 78,743 now U.S. Pat. No. 4811046 filed in the name of Jerome May and assigned to the same assignee as the instant application which relates to tri-level printing discloses apparatus wherein undesirable transient development conditions that occur during start-up and shut-down in a tri-level xerographic system when the developer biases are either actuated or de-actuated are obviated by the provision of developer apparatuses having rolls which are adapted to be rotated in a predetermined direction for preventing developer contact with the imaging surface during periods of start-up and shut-down. The developer rolls of a selected developer housing or housings can be rotated in the contact-prevention direction to permit use of the tri-level system to be utilized as a single color system or for the purpose of agitating developer in only one of the housings at a time to insure internal triboelectric equilibrium of the developer in that housing.
U.S. patent application Ser. No. 947,321 now U.S. Pat. No. 4771314 filed in the name of Parker et al and assigned to the same assignee as the instant application which relates to tri-level printing discloses printing apparatus for forming toner images in black and at least one highlighting color in a single pass of a charge retentive imaging surface through the processing areas, including a development station, of the printing apparatus. The development station includes a pair of developer housings each of which has supported therein a pair of magnetic brush development rolls which are electrically biased to provide electrostatic development and cleaning fields between the charge retentive surface and the developer rolls. The rolls are biased such that the development fields between the first rolls in each housing and the charge retentive surface are greater than those between the charge retentive surface and the second rolls and such that the cleaning fields between the second rolls in each housing and the charge retentive surface are greater than those between the charge retentive surface and the first rolls.
U.S. patent application Ser. No. 95,486 now U.S. Pat. No. 4833504 filed in the name of Delmer Parker and assigned to the same assignee as the instant application which relates to tri-level printing discloses a magnetic brush developer apparatus comprising a plurality of developer housings each including a plurality of magnetic rolls associated therewith. The magnetic rolls disposed in a second developer housing are constructed such that the radial component of the magnetic force field produces a magnetically free development zone intermediate a charge retentive surface and the magnetic rolls. The developer is moved through the zone magnetically unconstrained and, therefore, subjects the image developed by the first developer housing to minimal disturbance. Also, the developer is transported from one magnetic roll to the next. This apparatus provides an efficient means for developing the complementary half of a tri-level latent image while at the same time allowing the already developed first half to pass through the second housing with minimum image disturbance.
U.S. patent application Ser. No. 31,627 filed in the name of Parker et al and assigned to the same assignee as the instant application which relates to tri-level printing discloses an electronic printer employing tri-level xerography to superimpose two images with perfect registration during the single pass of a charge retentive member past the processing stations of the printer. One part of the composite image is formed using Magnetic Ink Character Recognition (MICR) toner, while the other part of the image is printed with less expensive black, or color toner. For example, the magnetically readable information on a check is printed with MICR toner and the rest of the check in color or in black toner that is not magnetically readable.
The problem of fringe field development in a tri-level highlight color, single pass imaging system is addressed in U.S. patent application (D/86201, now Ser. No. 07/132074 Attorney's Docket No.) mailed to the U.S. Patent Office on or about Dec. 9, 1987 which application is assigned to the same assignee as the instant invention.
In this application there is disclosed a magnetic brush developer apparatus comprising a plurality of developer housings each including a plurality of magnetic brush rolls associated therewith. Conductive magnetic brush (CMB) developer is provided in each of the developer housings. The CMB developer is used to develop electronically formed images. The developer conductively, as measured in a Gutman conductivity cell, is in the range of 10-9 to 10-13 (ohm-cm)-1. The toner concentration of the developer is in the order of 2.0 to 3.0% by weight and the charge level is less than 20 microcoulombs/gram and the developer rolls are spaced from the charge retentive surface a distance in the order of 0.40 to 0.120 inch.
U.S. patent application (D/87227, now Ser. No. 07/131498 Attorney's Docket No.) mailed to the U.S. Patent Office on or about Dec. 8, 1987 which application is assigned to the same assignee as the instant invention discloses a highlight color imaging method and apparatus including structure for forming a single polarity charge pattern having at least three different voltage levels on a charge retentive surface wherein two of the voltage levels correspond to two image areas and the third voltage level corresponds to a background area. Interaction between developer materials contained in a developer housing and an already developed image in one of the two image areas is minimized by the use of a scorotron to neutralize the charge on the already developed image.
U.S. Pat. No. 4,430,402 granted to Shuichi Tsushima on Feb. 7, 1984 discloses to two-component type dry developer for use in dichromatic electrophotography comprising two kinds of developers, wherein the developers comprises a toner and a carrier and are adapted to develop both positively and negatively electrified electrostatic images successively with toners different in polarity and color from each other and further wherein one carrier has a triboelectrification property of being electrified positively by friction with either of the two toners while the other carrier has a triboelectrification property of being electrified negatively by friction with either of the two toners.
The process of creating tri-level, highlight color images on a charge retentive sureface results in a charge retentive surface containing both positive and negative images which must be conditioned prior to transfer to a copy substrate. To this end, a positive pre-transfer corona discharge device is provided which changes the polarity of the negative image to positive and increases somewhat the polarity of the positive image. After transfer the residual toner remaining on the charge retentive surface is removed at a cleaning station. At the cleaning station, a cleaning member such as a magnetic brush is electrically biased to a negative polarity to enhance removal of positive residual toner. The brush portion of the magnetic brush is formed by means of carrier beads which extend in a radial direction form a base member. Heretofore, in an two-color imaging system of the type contemplated, some of the toner removed by the cleaning system was redeposited upon the charge retentive surface. This is an undesirable phenomenon. We discovered the cause of this redeposition to be attributable to the negative toner, which had been changed to a positive polarity by the pre-transfer step, having its polarity reversed in the cleaning apparatus. This is because of the triboelectric relationship between the carrier in the cleaner housing and this toner caused the toner to charge negatively through its interaction with the particular carrier employed.